The relation between dry vortex merger and tropical cyclone genesis over the Atlantic Ocean

Abstract

Using 33 year European Centre for Medium-Range Weather Forecasts Re-Analysis Interim reanalysis in the months of August and September, we found that more than half of the low-level, moist vortices (called wet vortices) originating from south of the African easterly jet merged with a shallow, dry vortex from the north after leaving the West African coast. A dry vortex involved with the merger process is referred to as a D-vortex, and the process is referred to as a D-vortex merger. Dry vortices influenced by more intense African easterly waves moved southwestward and had a greater potential to serve as D-vortices in the merger process. The D-vortex merger occurred in the predepression stage of 70% of tropical cyclones (TCs) that formed in the Atlantic main development region and in 55% of nondeveloping systems. Further analysis showed that developing systems with the D-vortex merger (DM) were statistically dominated by a more intense wet vortex whose 500 hPa relative humidity was also significantly higher, while nondeveloping systems with the D-vortex merger (NM) were dominated by a more intense dry vortex. The average intensity of wet vortices for DM was more intense than that for NM, significant at a 95% confidence level. Moreover, warmer Saharan air was observed for DM than NM. While TC genesis is largely controlled by the large-scale environment over ocean, differences in vortex characteristics and environment over northwestern Africa between DM and NM could potentially help predict whether a tropical system associated with the D-vortex merger will ultimately evolve into an Atlantic TC.